The present invention relates to refining discs and plate segments for refining discs, and more particularly to the shape of the bars that define the refining elements of the discs or segments.
Disc refiners for lignocellulosic material, ranging from saw dust to wood chips, are fitted with refining discs or segments. The material to be refined is treated in a gap defined between two refining discs rotating relative to each other. The material moves in the grooves formed by the bars located on the disc surfaces, both in a generally radial plane, providing a transport function, and out of plane, providing a mechanism for material stapling on the leading edges of the crossing bars. The instantaneous overlap between the bars located on each of the two disc faces forms the instantaneous crossing angle. The crossing angle has a vital influence on the material stapling or covering capability of the leading edges.
Conventional bar geometries, particularly parallel straight line, radial straight line, and curved in the form of inviolate arcs on circular evolutes, show a change of bar crossing angle with respect to radial position within refining zones. Parallel straight-line patterns show furthermore a change of bar angle with respect to peripheral position within a field of parallel bars.
Since bar crossing angle is a determining factor for covering probability, a variation in bar angle leads to a variation in covering probability as well. Therefore an inhomogeneous distribution of material in the gap as a function of radial and angular position is unavoidable by conventional bar designs. Representative patents directed to particular configurations of bars and grooves on segments for refiner plates, include: U.S. Pat. No. 6,276,622 (Obitz), “Refining Disc For Disc Refiners”, Aug. 21, 2001; U.S. Pat. No. 4,023,737 (Leider et al.), “Spiral Groove Pattern Refiner Plates”, May 17, 1977; and U.S. Pat. No. 3,674,217 (Reinhall), “Pulp Fiberizing Grinding Plate”, Jul. 4, 1972.